The field of art to which this invention pertains is polygalactomannans and derivatives thereof.
Derivatives of polygalactomannans, such as the hydroxyalkyl ether, alkyl ether, carboxyalkyl ethers, aminralkyl ether and quaternary ammonium alkyl ether derivatives, are well known compounds and various methods for preparing the derivatives have been described.
The hydroxyalkyl ethers of polygalactomannans are prepared by reacting the polygalactomannan with an alkylene oxide or an alkylene chlorohydrin under basic conditions. In U.S. Pat. Nos. 3,723,408 and 3,723,409, guar flour is reacted with alkylene oxides in the presence of water and sodium hydroxide. The reaction product is then neutralized with acid, washed with an alcohol-water mixture, and is then dried and ground. In U.S. Pat. No. 3,483,121, the polygalactomannan and the alkylene oxide are reacted under basic conditions with small amounts of water and larger amounts of water miscible or water immiscible organic solvents.
Carboxyalkyl ethers and mixed carboxyhydroxyalkyl ethers of polygalactomannans are described in U.S. Pat. Nos. 3,740,388 and 3,723,409, respectively. These derivatives are made by reacting the polygalactomannan with the derivatizing agents (halofatty acid and alkylene oxide) in a water-alcohol mixture followed by washing with a water-alcohol mixtures.
Other derivatives of polygalactomannans are described in such patents as U.S. Pat. No. 2,461,502 (cyanoethyl ethers), U.S. Pat. No. 4,094,795 (dialkylacrylamide ethers) and U.S. Pat. No. 3,498,912 (quaternary ammonium alkyl ethers). In the described processes, the reactions are conducted in water-organic solvent mixtures and the reaction products are washed with solvents or water-solvent mixtures.
In order to avoid the use of volatile flammable organic liquids and to eliminate the need to recover such organic liquids when the reactions are completed, commercial processes have been developed which use only water as the reaction medium. In such processes, the gum endosperm of the polygalactomannan is reacted with the derivatizing agent under alkaline catalysis using sufficient water to swell the endosperm. The resulting products are then washed to remove unreacted derivatizing agent, caustic, salt and by-products. During the washing step, care must be exercised to avoid forming gels which are extremely difficult to handle and to avoid washing away product.
The water washing problems have been minimized to a great extent by adding- borax at the end of the reaction or in the wash water. Borax, under basic pH conditions, will complex with polygalactomannans to form crosslinked gels. Small amounts of borax in the derivatizing reaction will complex and crosslink the surface of the swollen endosperm particles so that absorption of water and solubilization of the particles is inhibited. The use of borax increases the efficiency of the process. However, the disadvantage of the process is that the resulting polygalactomannan products have a slow hydration rate under high pH conditions.